Due to some email configuration issues, I opened Mozilla Thunderbird, not my regular email client, and noticed that its three panes (folders list, emails list, and message) were not completely opaque, showing a very small amount of background application windows bleed through. Mozilla Firefox usually resides beside itself on its own virtual desktop that has a neutral background color. When I moved it to the same desktop that had Thunderbird on it, I could see that Firefox also has that same small amount of background windows bleed through.

Of the various application windows that I have open, about 15, these are the only ones exhibiting this behaviour.

Currently, I can't find any KDE setting available to set windows' opacity. It's my habit to set windows' opacity to 100%, as I find windows transparency to be annoying.